Structure and method for self protection of power device with expanded voltage ranges

ABSTRACT

A vertical semiconductor power device includes a top surface and a bottom surface of a semiconductor substrate constituting a vertical current path for conducting a current there through. The semiconductor power device further includes an over current protection layer composed of a material having a resistance with a positive temperature coefficient (PTC) and the over current protection layer constituting as a part of the vertical current path connected to a source electrode and providing a feedback voltage a gate electrode of the vertical semiconductor power device for limiting a current passing there through for protecting the semiconductor power device at any voltage.

This application is a Divisional application of a co-pending applicationand claim the Priority Date of application Ser. No. 12/156,305 file by acommon Applicant of the application on May 31, 2008. Application Ser.No. 12/156,305 is a Continuation in Part (CIP) application and claimspriority to pending U.S. patent application entitled “STRUCTURE ANDMETHOD FOR SELF PROTECTION OF POWER DEVICE” filed on Jan. 25, 2007 andaccorded Ser. No. 11/657,862 and issued as U.S. Pat. No. 7,999,363 by acommon Applicant of this application, the benefit of its filing datebeing hereby claimed under Title 35 of the United States Code. Thedisclosures made in application Ser. Nos. 12/156,305 and 11/657,862 arehereby incorporated by reference in this application.

1. FIELD OF THE INVENTION

The invention relates generally to a device configuration and method ofprotecting the device. More particularly, this invention relates to animproved circuit configuration and method of protecting active devicesoperated at a high voltage from damages due to over current caused by anelectrical short by using a conductive material with a positivetemperature coefficient.

2. DESCRIPTION OF THE RELEVANT ART

Under high power conditions, the power devices often fail in a “shortcircuit” condition and that can result in damages to other components inthe system where the power devices are implemented. Therefore, it ispreferable that a power device fails in an open circuit state and evenmore preferable that a failure can be avoided entirely. Prevention offailure can be achieved by integrating a protective circuit as part ofthe device or outside of the device. One particular protectionimplemented in a semiconductor device is to place a fusible link such asa metal fuse or a polysilicon fuse shown in FIG. 1A in the interconnectpattern before the bond pads. However, as shown in FIG. 1B suchconfiguration has a limitation that for very high current application,multiple fuses and pads would be required thus causing an increase indie size and that leads to unfavorable cost impacts in manufacturing andin operation. Another disadvantage associates with multiple fuses andpads is that in case not all fuses blow open during an over-currentcondition, the active region of the device where the fuse did not blowopen would be damaged. Furthermore, the addition of the protectivecircuit such as metal fuse, poly-fuse, or other “fail-open” protectivecircuits, often adversely affects the device or system performance. Thefuse protection also has a disadvantage that such protection is notre-settable, once the fuse is broken, the fuse connection is broken eventhe over current condition is then eliminated.

In order to overcome such limitations, materials of a positivetemperature coefficient (PTC) are implemented as re-settable overcurrent protection device. Various PTC materials are known such as apolymer PTC (PPTC), which is commercially available in the market as“PolyFuse”, “Polyswitch” and “Multiswitch”. The products may be providedas a slice of plastic with carbon grains embedded in it. When theplastic is cool, the carbon grains are all in contact with each other,forming a conductive path through the device. When the plastic heats up,it expands, forcing the carbon grains apart, and causing the resistanceof the device to rise rapidly. Like the BaTiO3 thermistor, this devicehas a highly nonlinear resistance/temperature response and is used forswitching, not for proportional temperature measurement. The PTCapplication is disclosed in U.S. Pat. No. 4,238,812 and various datasheets that provide the FTC materials as commercial products.Effectiveness of PTC protections has been demonstrated with an increasein resistance of up to five orders of magnitude when the temperature isincreased.

Even though the application of materials of a positive temperaturecoefficient (PTC) for over-current protection of electronic devices iswell known, there are still technical limitations and difficulties whenthe FTC materials are actually implemented. The FTC protection circuitis usually achieved by connecting the protection device as in FIG. 1Ccomprising PTC a material that has a resistance increases withtemperature to a load. However, in order to achieve the protection,self-heating is usually required to increase the temperature and thatrequires an I.sup.2R drop within the device with a special mounting toavoid the heat sink that could reduce the effectiveness of deviceprotection. In the meantime, higher resistance can induce moreself-heating to achieve better protection. However, that negativelyimpacts the performance of the power systems. Alternatively, externalheating may be used to heat the PTC materials by adding more circuitcomponents but the added circuit components cause the current limitingprotection to occupy large volume.

Another difficulty encountered in the protection of device applying thePTC materials is in the device operated at higher voltage. For the sakeof reducing power consumption and increase operational efficiency, it isgenerally desirable to reduce the resistance of the protection circuits.However, as the resistance of the protection circuit is reduced, thevoltage rating of the protection also tends to reduce. For someapplication with devices necessary to operate at a high voltage rating,the reduction of voltage rating of the protection circuit implementedwith the materials with a positive temperature coefficient (PTC) becomesa major drawback and limitation. In addition, as the PTC protectioncircuit turns off, it takes on the applied (drain-source) voltage.Therefore, the voltage rating of the PTC device needs to equal or exceedthe applied voltage. For higher voltage applications, this requires aPTC device to have correspondingly high voltage rating, which increasesthe resistance of the PTC device, even when during normal operation.

Specifically, a patent application Ser. No. 11/657,862 filed on Jan. 25,2007 discloses a new device configuration, the disclosures ofapplication Ser. No. 11/657,862 is hereby incorporated by reference inthis application. The application Ser. No. 11/657,862 discloses deviceconfiguration that has a PTC protection layer connected in series withthe drain electrode of the active device. The device configurationhowever has a drawback that the voltage rating of the PTC structure hasto equal or exceed the applied voltages thus limiting the scopes ofapplications of this type of protection circuits.

Therefore, a need still exists in the fields of circuit design anddevice manufactures for providing a new and improved configuration andmanufacturing method to resolve the above-discussed difficulties.Specifically, a need still exists to provide new and improvedconfiguration to implement the FTC protection such that the abovediscussed limitations and difficulties can be resolved.

SUMMARY OF THE PRESENT INVENTION

It is therefore an aspect of the present invention to provide a new andimproved resettable PTC protection configuration that can work with anyenhancement mode device and any voltage rating such that theabove-discussed difficulties and limitations can be resolved.

It is an aspect of this invention that by applying the PTC (positivetemperature coefficient) fuse protection layer to the source connectionof the active device a PTC structure is inherently protected by thesystem or by the external circuit in this new configuration. The FTCfuse applies a feedback voltage to the FET gate control voltage thatautomatically turns the transistor off as an over-current and hightemperature condition occurs.

It is an aspect of this invention that the PTC fuse protection layer isstacked with the FET, such that the fuse is applied to the sourceconnection of the FET.

It is an aspect of this invention that by applying the PTC fuse to thesource connection of the active device wherein a standard top sourcevertical enhancement mode FET is implemented with the drain disposed atthe bottom of a semiconductor substrate. The FTC structure is formed onthe top of the FET to connect to the source electrode. The FTC structureis inherently protected by the system or by the external circuit in thisnew configuration because the FTC fuse applies a feedback voltage to theFET gate control voltage to turn off the transistor automatically.

It is an aspect of this invention that by applying the FTC fuse to thesource connection of the active device wherein a bottom source verticalenhancement mode FET is implemented with the source disposed at thebottom of a semiconductor substrate. The PTC structure is formed on thebottom of the FET to connect to the source electrode. Again, the PTCstructure is inherently protected by the system or by the externalcircuit in this new configuration because the PTC fuse applies afeedback voltage to the FET gate control voltage to turn off thetransistor automatically.

It is an aspect of this invention that by applying the PTC fuse to thesource connection of the active device, a temperature rise within theelectronic device is automatically imposed onto the PTC material andfurthermore, the PTC protection is implemented with minimum increase ofthe parasitic resistance and expanded voltage ranges such that theabove-discussed difficulties and limitations encountered by theconventional PTC protection can be overcome.

It is an aspect of this invention that by applying the PTC fuse to thesource connection of the active device that has improved thermalcoupling with the heat generation components or regions within anelectronic device and meanwhile, such coupling is provided with minimumincrease in coupling or parasitic resistance.

It is an aspect of this invention that by applying the PTC fuse to thesource connection of the active device to provide a new and improved PTCprotection configuration that the PTC protection configuration can beconveniently implemented with standard packaging techniques thus theunfavorable impact on manufacture costs can be avoided.

It is an aspect of this invention that by applying the PTC fuse to thesource connection of the active device to provide a new and improved FTCprotection configuration that the FTC protection configuration can beimplemented without increase in package size thus decreasing potentialunfavorable impact on manufacture costs.

It is an aspect of this invention that by applying the PTC fuse to thesource connection of the active device to provide a new and improved FTCprotection configuration that the PTC protection fuse layer composed aFTC material such as ceramic-based FTC materials or polymeric-based FTC(PPTC) materials.

Briefly in a preferred embodiment this invention discloses a verticalsemiconductor power device that includes a top surface and a bottomsurface of a semiconductor substrate constituting a vertical currentpath for conducting a current there through. The semiconductor powerdevice further includes an over, current protection layer composed of amaterial having a resistance with a positive temperature coefficient(PTC) and the over current protection layer constituting as a part ofthe vertical current path connected to a source electrode and providinga feedback voltage a gate electrode of the vertical semiconductor powerdevice for limiting a current passing there through for protecting thesemiconductor power device at any voltage. In an exemplary embodiment,the over current protection layer composed of a resettable currentlimiting material. In another exemplary embodiment, the over currentprotection layer is attached to the bottom surface. In another exemplaryembodiment, the over current protection layer is attached to the topsurface of the semiconductor power device with bonding wires connectedto the source electrode. In another exemplary embodiment, the overcurrent protection layer is disposed between the bottom surface of asemiconductor substrate constituting the source electrode of thesemiconductor power device and a leadframe.

The present invention further discloses a method for manufacturing anelectronic device with an integrated over current protection. The methodincludes a step of disposing an over current protection layer composedof a current limiting material for contacting a source electrode oft theelectronic device. The method further includes a step of disposing alayer composed of a material having a positive temperature coefficient(PTC) as the current limiting protection layer on a top surface of asemiconductor power device functioning as the source electrode of thesemiconductor power device. The method further includes a step ofdisposing a layer composed of a material having a positive temperaturecoefficient (PTC) as the current limiting protection layer on a bottomsurface of a semiconductor power device functioning as the sourceelectrode of the semiconductor power device.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment,which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C show different implementation configurations of overcurrent protections for protecting devices from damages caused bycircuit short.

FIG. 2 is schematic representation of a PTC protection circuit connectedto a source electrode.

FIG. 3A is a circuit diagram, FIG. 3B is a side cross sectional view andFIG. 3C is a top view of a standard vertical DMOS power MOSFET deviceimplemented with a FTC protection of this invention.

FIG. 4A is a circuit diagram, FIG. 4B is a side cross sectional view andFIG. 4C is a top view of a bottom source LDMOS power MOSFET deviceimplemented with a PTC protection of this invention.

FIGS. 5 to 12 show the side cross sectional views of differentembodiments of semiconductor power device protected by the PTC structureof this invention.

DETAILED DESCRIPTION OF THE METHOD

FIG. 2 is a schematic representation of a PTC protected FET 100, (whichcan also be referred to as a fuse FET) that includes a resistor 110composed of a material, that has a positive temperature coefficient(FTC) and configured as a conductive layer connected to a sourceelectrode and further provides a feedback voltage to regulate thegate-source voltage. As the temperature of the device under protectionis increased, the resistance of the PTC resistor 110 is also increasedand the resistance is drastically increased once the PTC triptemperature is reached. In this protection configuration, any source todrain current (Ids) increase will result in a voltage drop Vptc acrossthe PTC structure 110 thus reducing the FET control voltage Vgs.Specifically, the voltages have functional relationships that can berepresented as:

Vgs=Vgs′+Vptc

Vgs′=Vgs−Vptc

When the temperature of the device is above the trip temperature of thePTC, the voltage drop across the FTC structure Vptc is much greater thanthe gate to drain voltage Vgs′, i.e., Vptc>>Vgs′, and causes the FET toturn down or shut off. The trip temperature is selected to protect thefuse FET from over current conditions. Therefore, the voltage changes ofthe FET under protection automatically protect the PTC structure fromover-voltage by itself. Furthermore, this fuse FET configuration canalso be applied to any drain voltages as well with the only requirementthat the FTC structure should match the gate-source voltage rating ofthe FET under protection by the FTC structure, i.e., the voltage ratingof the FTC structure should be as great or greater than the gate-sourcevoltage rating of the FET. The voltage rating of the PTC structure isindependent, of the drain (drain-source) voltage. Even in high voltageapplications (high drain-source voltage), a PTC structure can have a lowvoltage rating, and thus provide low resistance for the fuse FET undernormal operating conditions.

FIG. 3A is a schematic circuit diagram and FIGS. 3B to 3C illustrate theactual implementation of such PTC protection structure for a verticalDMOS with a top source and bottom drain configuration. A FET device 100′is mounted on a printed circuit board (PCB) 160 and connected throughleads of a lead frame 150 with a gate connection 150-G and sourceconnection 150-S disposed on top of the PCB. A vertical DMOS device witha bottom drain electrode 140 is disposed on top of the lead frame 150and has a top source metal 120. A PTC protection structure that includesa PTC layer 110′-PTC and a top and bottom electrode layers 110′-e1 and110′-e2 respectively are disposed on top of the source metal layer 120.The top and bottom electrodes of the PTC devices can be composed of anygold containing metal, alloy, or multi-layer structure such as Au, NiAu,etc., or copper (Cu). The PTC layer 110′-PTC may include a FTCprotection fuse layer composed of ceramic-based PTC materials orpolymeric-based PTC (PPTC) materials, or any other suitable PTCmaterial. Bonding wires 125 connected the top electrode 110′-e1 to theleadframe 150-S. The bonding wires 125-G connected between the gate pad130 and the leadframe 150-G. The PTC structure is attached on the activesource regions of the FET; The PTC structure, DMOS, bonding wires andparts of the leadframe are encapsulated in a molding compound to form asemiconductor package.

FIG. 4A is a schematic circuit, diagram and FIGS. 4B to 4C illustratethe actual implementation of such PTC protection structure for avertical DMOS with a bottom source and top drain configuration. A FETdevice 100″ is mounted on a printed circuit board (PCB) 160 andconnected through leads of a lead frame 150 with a gate connection 150-Gand drain connection 150-D disposed on top of the PCB 160. A verticalDMOS device with a bottom source electrode 120′ disposed on top of thePTC protection structure 110′ disposed on top of the leadframe 150. ThePTC structure comprises a PTC layer 110′-PTC padded by a top and bottomelectrode layers 110′-e1 and 110′-e2′ respectively. The top and bottomelectrodes of the PTC devices can be composed of any gold containingmetal, alloy, or multi-layer structure such as Au, NiAu, etc., or ofcopper (Cu). The bonding wires 125-D connect the top drain contact metal140′ formed on top of the VDMOS device with a bottom source 120′ to theleadframe 150-D. The bonding wires 125-G connects the gate pad 130 tothe leadframe 150-G. In this FET device, the PTC electrode 110′-e1 is inimmediate contact with the bottom source electrode 120′, and the PTCelectrode 110′-e2 is in immediate contact with the leadframe 150.

The basic configuration of connecting the PTC structure to the sourceelectrode as shown in FIGS. 3A to 4C can be implemented for deviceprotection for any voltage range without being limited by the maximumvoltage ratings of the PTC structures as the limitations encountered inthe PTC to drain connection protection configurations. Furthermore, theprotection configurations are also applicable to any enhancement modeFET devices, which include the low-voltage (LV) trench gate VDMOS FETs,the planar VDMOS FETs, the E-mode HEMTs (high electron mobilitytransistors), the E-Mode SiT (static induction transistor) and JFETs(junction gate FET). In addition to the wire-bonding package as shown,plate bonding can also, be implemented to improve performance.

FIG. 5 is a cross sectional view for showing a standard vertical DMOS(VDMOS) power MOSFET die 200. A VDMOS device 200 is mounted on a printedcircuit board (PCB) 260 and connected through the leads of a lead frame250 with a gate connection 250-G and source connection 250-S disposed ontop of the PCB. The vertical DMOS device 200 has a bottom drainelectrode 240 disposed on top of the lead frame 250 and a top sourcemetal 220 such as an aluminum source contact layer. Anoxidation-resistant layer formed by an electroless NiAu plating may beformed on the aluminum surface prior to the attachment of the PTCprotection structure 210. The drain electrode is formed by a standardback metal process on the MOSFET die using a standard back metal layersuch as a TiNiAg or TiAu or CrAu layer to form the drain electrode asthe bottom electrode 240. The PTC protection structure includes a PTClayer 210-PTC and a top and bottom electrode layer 210-e1 and 210-e2.The electrode-layers 210-e1 and 210-e2 can be formed with NiAu pressedonto the PTC layer 210-PTC from the top and bottom respectively. Thus,the top and bottom electrodes of the PTC devices can be composed of anygold containing metal, alloy, or multi-layer structure such as Au, NiAu,or of copper (Cu). Bonding wires 225 connect the top electrode 210-e1 tothe leadframe connection 250-S. The bonding wires 225-G connectedbetween the gate pad 230 and the leadframe connection 250-G. The PTCstructure is attached on the active source regions of the FET. Thepackaging processes begin with the application of a conductive epoxy toattach the MOSFET device to the leadframe 250. Then, a conductive epoxyis applied on top of the source metal layer 220 for attaching the PTCstructure 210 on the top surface of, the source metal layer 220. A wirebonding process is processed for bonding the source bonding wire 225-Sconnecting the PTC structure 210 to the leadframe 250-S and gate bondingwire 225-G connecting the gate pad 230 to the leadframe 250-G. Thepackaging processes are completed by a step of encapsulating the devicein a molding compound 270 for encapsulating and protecting the VDMOSdie.

FIG. 6 is another standard VDMOS package 200′ similar to the deviceshown in FIG. 5, except that the packaging processes begin with theapplication of a solder paste rather than a conductive epoxy to attachthe MOSFET device to the leadframe 250. Then, a solder paste is appliedon top of the source metal layer 220 for attaching the PTC structure 210on the top surface of the source metal layer 220. A wire bonding processis processed for bonding the source bonding wire 225-S connecting thePTC structure 210 to the leadframe 250-S and gate bonding wire 225-Gconnecting the gate pad 230 to the leadframe 250-G. The packagingprocesses are completed by a step of encapsulating the device in anencapsulation. 270 for protecting the VDMOS die. FIG. 7 is anotherstandard VDMOS package 200″ similar to the device shown in FIG. 6 exceptthat a glob top 255 is formed covering the MOSFET device, PTC structure210, and parts of the bonding wires 225 and leadframe 250 after thecompletion of the wire bonding process, but before the encapsulationstep. The glob top 255 allows for thermal expansion of the FTC structure210, formed on the top, surface of the die.

FIG. 8 is a cross sectional view for showing a bottom source LDMOS die300. The bottom source LDMOS device 300 is supported on a printedcircuit board (PCB) 360 and is disposed on top of a lead frame 350 andconnected to the lead frame 350 with a gate connection 350-G and drainconnection 350-D disposed on top of the PCB. A vertical LDMOS devicewith a bottom source electrode 320 disposed on top of the PTC protectionstructure 310 disposed on top of the leadframe 350. A PTC protectionstructure that includes a PTC layer 310-PTC and a top and bottomelectrode layer 310-e1 and 310-e2 with NiAu pressed onto the PTC layer320 from the top and bottom respectively. Thus, the top and bottomelectrodes of the PTC devices can be composed of any gold containingmetal, alloy, or multi-layer structure such as Au, NiAu, or of copper(Cu).

The bonding wires 325-D connect the top drain contact metal 340 formedon top of the bottom source LDMOS device to the leadframe 350-D. Thebonding wires 325-G connects the gate pad 330 to the leadframe 350-G. Inthis bottom source LDMOS device, the PTC structure 310 is in immediatecontact with the bottom source electrode 320. The packaging processesbegin with the application of a solder paste to attach the PTC 310 tothe leadframe 350. Then, a solder paste is applied on top of the PTCstructure 310 for attaching the source electrode 320 of the MOSFETdevice onto the PTC structure 310. A wire bonding process is processedfor bonding the source bonding wire 325-D connecting the drain electrode340 to the leadframe 350-D and gate bonding wire 325-G connecting thegate pad 330 to the leadframe 350-G. The packaging processes arecompleted by a step of encapsulating the device in an encapsulation 370for protecting the bottom, source LDMOS die. The packaging processes forthe device as shown in FIGS. 8 and 9 may further be modified by forminga glob top after the completion of the wire bonding process before thefinal step of encapsulation such that more thermal expansion flexibilityof the FTC structure 310 is provided.

FIG. 9 is a cross sectional view for showing a bottom source LDMOS die300′. The bottom source LDMOS device 300 is supported on a printedcircuit board (PCB) 360 on top of a lead frame 350 and connected to alead frame with a gate connection 350-G and drain connection 350-Ddisposed on top of the PCB. A vertical LDMOS device with a bottom sourceelectrode 320 disposed on top of the PTC protection structure 310disposed on top of the leadframe 350. A PTC protection structure thatincludes a PTC layer 310-FTC and a top and bottom electrode layer 310-e1and 310-e2 with NiAu pressed onto the PTC layer 320 from the top andbottom respectively. Thus, the top and bottom electrodes of the PTCdevices can be composed of any gold containing metal, alloy, ormulti-layer structure such as Au, NiAu, or of copper (Cu). The bondingwires 325-D connect the top drain contact metal 340 formed on top of thebottom source LDMOS device, with a bottom source 320, to the leadframe350-D. The bonding wires 325-G connects the gate pad 330 to theleadframe 350-G. In this bottom source LDMOS device, the PTC structure310 is in immediate contact with the bottom source electrode 320. Thepackaging processes begin with the application of a conductive epoxy toattach the FTC 310 to the leadframe 350. Then, a conductive epoxy layeris applied on top of the FTC structure 310 for attaching the sourceelectrode 320 of the MOSFET device onto the PTC structure 310. A wirebonding process is processed for bonding the source bonding wire 325-Dconnecting the drain electrode 340 to the leadframe 350-D and gatebonding wire 325-G connecting the gate pad 330 to the leadframe 350-G.The packaging processes are completed by, a step of encapsulating thedevice in an encapsulation 370 for protecting the bottom source LDMOSdie. The packaging processes for the device as shown in FIGS. 8 and 9may further be modified by forming a glob top after the completion ofthe wire bonding process before the final step of encapsulation suchthat more thermal expansion flexibility of the FTC structure 310 isprovided.

FIG. 10 is a cross sectional view for showing a standard vertical DMOS(VDMOS) power MOSFET die 400. A VDMOS device 400 is supported on aprinted circuit board (PCB) 460 and connected to a lead frame with agate connection 450-G and source connection 450-S disposed on top of thePCB. The vertical DMOS device 400 has a bottom drain electrode 440disposed on top of the leadframe 450 and a top source metal 420 such asan aluminum source contact layer. An oxidation-resistant layer formed byan electroless NiAu plating may be formed on the exposed aluminumsurface. The drain electrode is formed by standard back metal process onthe MOSFET die using TiNiAg or TiAu or CrAu layer to form the drainelectrode as the bottom electrode 440. A PTC protection structure thatincludes a PTC layer 410-PTC and a top and bottom electrode layer 410-e1and 410-e2 made from NiAu pressed onto the PTC layer 410-PTC from thetop and bottom respectively. A top plate 425-S-P connects the topelectrode 410-e1 to the leadframe connection 450-S. The bonding wires425-G connect between the gate pad 430 and the leadframe connection450-G. The PTC structure is attached on the active source regions of theFET. The packaging processes begin with the application of a solderpaste to attach the MOSFET device to the leadframe 450. Then, a solderpaste is applied on top of the source metal layer 420 for attaching thePTC, structure 410 on the top surface of the source metal layer 420.Alternatively, instead of solder paste, a similarly functioning materialcan be used, e.g., conductive epoxy. A top plate bonding process isprocessed for bonding the source top plat 425-S-P connecting the PTCstructure 410 to the leadframe 450-S and gate bonding wire 425-Gconnecting the gate pad 430 to the leadframe 450-G. The packagingprocesses are completed by a step of applying an elevate temperature forreflow and encapsulating the device in an encapsulation 470 forprotecting the VDMOS die then cutting the leadframe and tabs or saw thewafer into VDMOS MOSFET dies. FIG. 11 shows another standard VDMOSMOSFET power device 400′ similar to the semiconductor power device 400as shown in FIG. 10. The only difference is that the gate bonding wires425-G shown in FIG. 10 is now replaced with a gate top plate 425-G-Pconnecting between the gate pad 430 to the leadframe gate connection450-G.

FIG. 12 is a cross sectional view for showing a bottom source LDMOS die500. The bottom source LDMOS device 500 is supported on a printedcircuit board (PCB) 560 under a leadframe 550. The leadframe 550 has agate connection 550-G and a drain connection 550-D and is disposed ontop of the PCB 160. A vertical LDMOS device with a bottom sourceelectrode 520 is disposed on top of the PTC protection structure 510,which is disposed on top of the leadframe 550. A PTC protectionstructure includes a PTC layer 510-PTC and a top and bottom electrodelayer 510-e1 and 510-e2 which can be made from NiAu pressed onto the PTClayer 510-PTC from the top and bottom respectively. Thus, the top andbottom electrodes of the PTC devices can be composed of any goldcontaining metal, alloy, or multi-layer structure such as Au, NiAu, orof copper (Cu). The top bonding plate 525-D-P connect the top draincontact metal 540 formed on top of the bottom source LDMOS device, witha bottom source 520, to the leadframe 550-D. The gate bonding plate325-G connects the gate pad 530 to the leadframe 550-G. In this bottomsource LDMOS device, the PTC structure 510 is in immediate contact withthe bottom source electrode 520. The packaging processes begin with theapplication of a solder paste to attach the PTC 510 to the leadframe550. Then, a solder paste is applied on top of the PTC structure 510 forattaching the source electrode 520 of the MOSFET device onto the PTCstructure 510. Alternatively a similarly suitable material such asconductive epoxy can be used instead of solder. A plate bonding processis processed for bonding the drain top plate 525-D-P connecting thedrain electrode 540 to the leadframe 550-D and gate bonding plate525-G-P connecting the gate pad 530 to the leadframe 550-G. Thepackaging processes are completed by a step of applying an elevatetemperature to allow solder reflow and encapsulating the device in anencapsulation 570 for protecting the bottom source LDMOS die.

The FTC layers shown in the above FIGS. 4A to 12, such as the PTC layers110′-FTC, 210-PTC, 310-FTC, 410-FTC and 510-FTC, may include a PTCprotection fuse layer composed of either ceramic-based PTC materials orpolymeric-based FTC (PPTC) materials or any other suitable PTC material.These PTC materials may include doped ceramic of barium titanate as thatdisclosed in U.S. Pat. No. 4,238,812 even though there is a concern thatthe materials may have limited application due to the fact that thelayer may become brittle at a higher temperature. The FTC materials mayfurther include conductive polymers that have particulate conductivefiller dispersed in the bulk of the polymer layer as that disclosed in.U.S. Pat. No. 4,238,812 and have been successfully implementedcommercially by Raychem and other companies to fabricate FTC fuse layerwith a thickness of about 0.5 Angstroms available as a Trademarkedproduct under the Trademark of PolyFuse™.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter reading the above disclosure. Accordingly, it is intended that theappended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

We claim:
 1. A method for protecting a vertical semiconductor powerdevice having a top surface and a bottom surface of a semiconductorsubstrate constituting a vertical current path for conducting a currenttherethrough comprising: disposing an over current protection layercomposed of a material having a positive temperature coefficient (PTC)of resistance constituting as a part of said vertical current path andconnected to a source electrode and providing a feedback voltage to agate electrode of the vertical semiconductor power device for limiting acurrent passing there through for over-current protection not limited bya voltage rating of said over current protection layer.
 2. The method ofclaim 1 wherein: said step of disposing said over-current protectionlayer further comprising a step of disposing said over-currentprotection layer composed of a resettable current limiting material. 3.The method of claim 2 wherein: said step of disposing said over-currentprotection layer further comprising a step of attaching saidover-current protection layer on a bottom surface of a bottom-sourcesemiconductor power device for directly contacting a bottom source ofthe semiconductor power device.
 4. The method of claim 2 wherein: saidstep of disposing said over-current protection layer further comprisinga step of attaching said over-current protection layer on a top surfaceof a bottom-drain semiconductor power device for directly contacting atop source electrode of the semiconductor power device.
 5. The method ofclaim 2 wherein: said step of disposing said over-current protectionlayer further comprising a step of disposing said over currentprotection layer with a voltage rating matched with a gate-sourcevoltage rating of the semiconductor power device.